The present invention relates to cable designs for optical fiber cables, and more particularly to novel combinations of materials useful in the fabrication of such cables to facilitate the installation of optical fibers into the cables, after the cables have been installed, by the process known in the art as fiber blowing.
The terms "blow," "blown," and "blowable" as currently used in optical fiber design and optical telecommunications technology refer to optical fibers or lightweight optical fiber cables which may be installed in specially designed conduits or ducts by the effect of viscous fluid drag. The fibers or cables arc sufficiently small and/or light, and the ducts sufficiently close in size to the fibers or cables, that significant forces tending to draw the fibers or cables through the ducts can be generated by forcing a gas such as air through the ducts and past the fibers or cables in the direction in which they are to be drawn.
Optical fiber transmission media utilizing blown fibers and methods for installing them are well known. U.S. Pat. No. 4,796,790 describes some optical fiber transmission media useful for installation in this manner, while published European Application EP 0157610 discloses optical fiber coating designs for fibers for blow-in installation.
Published British Patent Applications GB 2,206,420 and 2,206,220 describe still other designs for blowable optical fibers and cables. The latter patent, particularly, discloses a design for a combination coating for a blowable optical fiber or cable which comprises a soft, low modulus inner coating supporting a foamed polymer outer sheath designed to improve the blowing characteristics of the fiber or cable.
The use of a foamed or roughened outer coating on conventionally coated optical fiber or lightweight cable provides significant performance advantages for blow-in applications. Hence, such a coating significantly increases the drag forces generated by gas flow past the fiber during blow-in installation.
However, in this type of fiber as well as in other blow-in designs, another critical factor to be considered is that of the frictional interaction between the fiber or cable to be installed and the plastic tube or microduct into which installation is to be accomplished. Quite apparently, it would be desirable to minimize the friction between the fiber or cable and the duct to be used in order to decrease sidewall drag and thus increase the duct distance through which fiber may be blown.
The plastic installation tube or duct has been recognized as a critical and integral part of the blown fiber system, since it provides the pneumatic pathway through which the blown fiber must be installed. The tube material composition must thus be such that the frictional interaction between the tube and the fiber units, which may be single fibers or small fiber bundles or minicables incorporating smooth or textured plastic coatings, is minimized. The lower the frictional interaction, the longer the duct length through which the fiber can be installed.
In addition to low friction characteristics, however, it is desirable that the tube or duct material meet or exceed NEC and UL flammability requirements for in-building use. Although the tubes are normally only a part of a larger cable system, they should not contribute to or aggravate the flame or smoke characteristics of the cables in the event of fire.
The art has identified a number of different materials as useful for fabricating tubes or ducts through which fibers or small cables may be installed. One early blown fiber duct system is found in British Patent Application GB 2,156,837A. That application discloses the use of plastic ducts, formed for example of polyethylene, into which a multipurpose additive is incorporated. The multipurpose additive comprises, among other agents, one or more ingredients which function as a slip agent, as an antistatic agent, and as an antioxidant.
In the approach outlined in this patent, the slip agent used may be any organic chemical which is incompatible with the polymer forming the duct wall. Due to this incompatibility, the additive blooms onto the inner surface of the polyethylene duct, and there acts as a lubricant to reduce friction between the wall and fibers being drawn through the duct.
One disadvantage of the approach of this patent, however, is that the slip characteristics of the duct do not remain constant over the projected storage and use life of the cable. The process of blooming is difficult to control and, moreover, can result in depletion of the slip agent even before the cable is used. Thus the slip agent is subject to loss by leaching or other transport mechanism, due to moisture or other other environmental factors, both during storage and while in use.
U.S. Pat. No. 4,691,896 also discloses designs for optical fibers and optical fiber ducts useful for blow-in installation of the fibers. In that patent, polyethylene is suggested for use as a duct wall material while polyethylene or polypropylene are used for the fiber coatings.
In the cable designs shown in the patent, the duct sizes disclosed are typically below 10 millimeters in diameter, and the fiber or cable sizes are in the range of 1-4 millimeters in diameter. Ducts in this size range are sometimes referred to as ductlets or microducts in the art.
New cable designs for electrical power lines which incorporate ducts for the blow-in installation of optical fibers are described by H. G. Haag et al. in "New Generation of Self-Supporting Optical Fibre Aerial Cables," International Wire and Cable Symposium Proceedings-1989, pages 575-582. The ducts used in the Haag et al. cable designs are typically composed of high density polyethylene or polyamide plastics.
Although the polymers used for duct wall fabrication up to the present time can permit fiber installation over reasonable distances, further improvements in the duct materials would be highly desirable. Thus, for example, significant reductions in the coefficient of friction between the duct material and the fiber coatings currently being used could significantly increase the lengths of optical fiber which could be installed therein.
At the same time, many of the polymers proposed for use as a duct material in the prior art exhibit relatively high flammability. Cables comprising significant proportions of these polymers are expected to have difficulty meeting current flammability requirements for in-building use.
Of course, a duct composition material offering improved chemical stability in storage and use would be advantageous. This characteristic is particularly important where cables are installed with empty ducts to satisfy existing telecommunications demand, but with the intention to add optical fiber links at an indefinite future date as demand increase warrant.
It is therefore a principal object of the present invention to provide designs for optical cables for blow-in fiber installation which provide highly stable, low friction performance.
It is a further object of the invention to provide designs for optical cables for blow-in fiber installation which offer an improved combination of low friction against existing fiber coating materials with improved performance against existing fire codes.
It is a further object of the present invention to provide a combination of an optical fiber and plastic duct material which exhibits improved flame retardance.
Other objects and advantages of the invention will become apparent from the following description thereof.